Studies on the application of alteration in genome size to rice breeding

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In order to obtain the diploid rice plants with altered nuclear DNA content, populations of artificial autotetraploids of rice ( Oryza sativa L. cv. Nipponbare and Fukunishiki ) were repeatedly exposed to gamma-rays through several generations. Plants which did not differ in appearance from the original diploid plants occurred occasionally in the populations. So far, 9 diploid plants were obtained. These plants were advanced their generations without irradiation in order to examine the segregation of characteristics and the variation of nuclear DNA content.In the experiments, it was revealed that the mutant strains obtained through tetraploid-irradiation technique had wider range of variation in nuclear DNA content than the mutant strains obtained by irradiation of diploid plant (Norin 8) or Japanese cultivares had. In addition, the direction of changes in DNA amount from those of original cultivares (Nipponbasare and Fukunioshiki) was both directional, whereas many of the mutant strai … More ns obtained by irradiation of diploid plants (Norin 8) reduced their DNA amount.In the investigation of mutated characters, it was indicated that several mutations of qualitatively and quantitatively inherited characters were induced in a single plant simultaneously through the tetraploid-irradiation technique. We examined the correlations among those mutated characters and also between DNA amount and the characters. It is well known that the plant height changes simultaneously with several agronomic characters such as number of spikelet per panicle, grain weight and so on. It is reported that these findings were interpreted as pleiotropic effect of a mutant gene, and these side effects were usually negative. The present results, however, suggest that these characters can be altered independently by using tetraploid-irradiation technique.The mutated characters were not correlated with DNA amount. The findings suggest that the genic mutations have more marked effects on phenotype than the genome size have. We considered that the genic systems are also altered through chromosome rearengement in the process of changing DNA amount. Further investigations will reveal the mechanisms involved. Less